This invention relates to a mobile telephone device, which may be a portable radio telephone device, namely, a portable telephone device for producing a radio output signal carrying an audio or audio-frequency signal.
Such a portable telephone device comprises a microphone for producing a talk or voice signal in response to a talk or speech spoken thereto by a user of the portable telephone device. While carried by the user, the portable telephone device is inevitably used in a noisy place where noise has an appreciably high noise level as a surrounding noise. Under such circumstances, the microphone produces an audio signal which comprises a talk signal component obtained from the talk signal and a noise component resulting from the surrounding noise.
It is well known in the art of telephony to use a compressor circuit on a transmitting side in order to raise a signal-to-noise (S/N) ratio. A compressor circuit is consequently used in the portable telephone device to produce a compressed signal by subjecting the audio signal to amplitude compression. A radio transmission arrangement transmits the compressed signal as a radio output signal to a base station. Receiving the radio output signal as a radio reception signal, the base station sends the radio reception signal after amplitude expansion to a counterpart substation which may either be one of fixed telephone units or substations or be one of similar other portable telephone devices.
In the manner which will later be described more in detail, the compressor circuit is for subjecting a circuit input signal to the amplitude compression and for thereby producing the compressed signal. In a conventional portable telephone device, the audio signal alone is supplied to the compressor circuit as the circuit input signal. Despite use of the compressor circuit in the portable telephone device and of an expander circuit in the base station, the noise component is unavoidably transmitted to the counterpart substation to be heard as noise by an attendant to the counterpart substation particularly during a pause in the talk. This annoys the attendant.
On the other hand, a cellular-type radio communication system is disclosed in U.S. Pat. No. 4,025,853 issued to Eric John Addeo and assigned to Bell Telephone Laboratories, incorporated. The radio communication system is for a plurality of mobile telephone devices, each of which may not necessarily be a motor-vehicle-mounted unit but may be a portable telephone device of the type described. At any rate, the mobile telephone device are for use in a service area of a base station.
In the Addeo patent, the base station is called a mobile telecommunication switching office. The service area is called a mobile communication area and is covered by a honeycomb type of cellular overlay which is divided into cell sets. Each cell set consists of several cells, such as seven cells. A predetermined number of communication channels are used in common to the cells of the cell sets by a cell site or station in each cell.
According to Addeo, each cell site transmits some communication channels with an individual supervisory audible tone (SAT) of about 6 kHz superposed thereon as a unique tone. Herein, one of the cell sites will be called a particular site. One of the mobile telephone devices will be called a particular device, it will be assumed that this one of the mobile telephone devices is used in the cell in which the particular site is.
The particular device receives one of the communication channels as a received signal and separates the supervisory audible tone as a separated audible tone from the received signal. Using the separated audible tone, the particular device transmits a verification signal carrying a reference tone which is related to the separated audible tone. Responsive to the verification signal, the particular site judges whether or not the separated audible tone is coincident with the unique tone.